1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Conventionally, a color-image forming apparatus using an electrophotographic system has performed image formation by reflecting a laser beam modulated in response to an image signal from a polygon mirror serving as a rotating polygon mirror and scanning a photosensitive drum with the reflected laser beam to form an electrostatic latent image. In such an image forming apparatus, when a plurality of scanners including light sources for laser beams in colors is used, a main body of the apparatus increases in size and the apparatus increases in cost.
To miniaturize the apparatus and reduce the cost thereof, a configuration in which a common scanner is used for a plurality of colors has been known. Furthermore, Japanese Patent Application Laid-Open No. 4-313776 discusses using a common scanner for a plurality of colors and providing a beam detect (BD) sensor for one of a plurality of light sources.
Japanese Patent Publication No. 4393133 discusses measuring, if an error α occurs on each of mirror surfaces of a polygon mirror (see FIG. 8A), a period of a BD signal when the mirror surface of the polygon mirror is scanned with a laser, using a BD sensor, and calculating a correction value for each mirror surface to correct the error.
By adding the correction value, it is possible to generate an accurate BD signal for light sources other than a light source provided with the BD sensor even if a plane division error occurs on each of the mirror surfaces of the polygon mirror. The BD signal for the light source other than the light source provided with the BD sensor is referred to as a pseudo BD signal.
In a configuration in which the pseudo BD signal is generated, discussed in Japanese Patent Publication No. 4393133, a scanner is started after a print instruction is received so that the polygon mirror converges at a rotation speed for performing image formation (a steady rotation state), and a correction value for generating the pseudo BD signal is then calculated. Then, the image formation is started. Thus, the start of the image formation is delayed by a period of time during which the correction value for generating the pseudo BD signal is calculated compared with a configuration in which the pseudo BD signal need not be corrected. As a result, a first print output time serving as a period of time that has passed until image formation on the first recording medium is completed since the print instruction has been received is extended.